[LeetCode] Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7
return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

[Solution].java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> all = new ArrayList<ArrayList<Integer>>();
        Stack<TreeNode> current = new Stack<TreeNode>();
        Stack<TreeNode> next = new Stack<TreeNode>();
        boolean order = true;
        if(root != null) current.push(root);
        while(!current.isEmpty()){
            ArrayList<Integer> one = new ArrayList<Integer>();
            while(!current.isEmpty()){
                TreeNode node = current.pop();
                one.add(node.val);
                if(order){
                    if(node.left != null) next.push(node.left);
                    if(node.right != null) next.push(node.right);
                }
                else{
                    if(node.right != null) next.push(node.right);
                    if(node.left != null) next.push(node.left);
                }
            }
            all.add(one);
            Stack<TreeNode> swap = current;
            current = next;
            next = swap;
            next.clear();
            order = !order;
        }
        return all;
    }
}